Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Recently, efforts have been devoted to identifying ways to operate electrochemical fuel cells using other than pure hydrogen as the fuel. Fuel cell systems operating on pure hydrogen are generally disadvantageous because of the expense of producing and storing pure hydrogen gas. In addition, the use of liquid fuels is preferable to pure, bottled hydrogen in mobile and vehicular applications of electrochemical fuel cells.
Recent efforts have focused on the use of impure hydrogen obtained from the chemical conversion of hydrocarbon fuels to hydrogen. However, to be useful for fuel cells and other similar hydrogen-based chemical applications, hydrocarbon fuels must be efficiently converted to relatively pure hydrogen with a minimal amount of undesirable chemical byproducts, such as carbon monoxide.
Conversion of hydrocarbons to hydrogen is generally accomplished through the steam reformation of a hydrocarbon such as methanol in a reactor sometimes referred to as a reformer. The steam reformation of methanol is represented by the following chemical equation: EQU CH.sub.3 OH+H.sub.2 O+heat.revreaction.3H.sub.2 +CO.sub.2 ( 1)
Due to competing reactions, the initial gaseous mixture produced by steam reformation of methanol typically contains from about 0.5% to about 20% by volume of carbon monoxide and about 65% to about 75% hydrogen, along with about 10% to about 25% carbon dioxide, on a dry basis (in addition, water vapor can be present in the gas stream). The initial gas mixture produced by the steam reformer can be further processed by a shift reactor (sometimes called a shift converter) to reduce the carbon monoxide content to about 0.2% to about 2%. The catalyzed reaction occurring in the shift converter is represented by the following chemical equation: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 2)
Even after a combination of steam reformer/shift converter processing, the product gas mixture will have minor amounts of carbon monoxide and various hydrocarbon species, each present at about 1% or less of the total product mixture. A variety of conventional treatment processes may be employed to remove many of the hydrocarbon and acid gas impurities generated during the steam reformer/shift converter process. However, such conventional treatment methods are generally incapable of reducing the carbon monoxide content of the gases much below about 0.2%.
In low-temperature, hydrogen-based fuel cell applications, the presence of carbon monoxide in the inlet hydrogen stream, even at the 0.1% to 1% level, is generally unacceptable. In solid polymer electrolyte fuel cells, the electrochemical reaction is typically catalyzed by an active catalytic material comprising a noble metal such as platinum. Carbon monoxide adsorbs preferentially to the surface of platinum, effectively poisoning the catalyst and significantly reducing the efficiency of the desired electrochemical reaction. Thus, the amount of carbon monoxide in the hydrogen-containing gaseous mixture produced by a steam reformer/shift converter process for use in electrochemical fuel cells should be minimized, preferably to amounts significantly lower than the approximately 1% achieved using conventional steam reformation methods. The present selective oxidizing method and apparatus reduce the amount of carbon monoxide in a hydrogen-containing gas stream to a level suitable for use in electrochemical fuel cells.
In the present selective oxidizing method and apparatus, it is believed that at least three competing reactions occur, which are represented by the following chemical equations:
1. The desired oxidation of carbon monoxide to carbon dioxide: EQU CO+1/2O.sub.2 .revreaction.CO.sub.2 ( 3) PA1 2. The undesired oxidation of hydrogen to water: EQU H.sub.2 +1/2O.sub.2 .revreaction.H.sub.2 O (4) PA1 3. The undesired reverse water-shift reaction: EQU CO.sub.2 +H.sub.2 .revreaction.H.sub.2 O+CO (5) PA1 effective turbulent flow over the catalyst pellets; PA1 containment of the catalyst pellets in a pressurized gaseous environment PA1 efficient heat transfer to and from the catalyst pellets; and PA1 addition and mixing of oxygen in the latter part of the reaction chamber containing the catalyst. PA1 the aluminum reactor housing is light weight and provides effective thermal conductivity; PA1 the stainless steel shell is able to withstand the stress developed by the pressurized gases within the reactor and also resists corrosion, both of which are particularly problematic at elevated temperatures. PA1 introducing a first amount of oxygen-containing gas into the gaseous mixture; PA1 contacting the gaseous mixture with a catalyst in a reaction chamber having an inlet, the catalyst promoting the oxidation of carbon monoxide to carbon dioxide; and PA1 introducing a further amount of oxygen or an oxygen-containing gas mixture into the reaction chamber through at least one secondary inlet located between the inlet and the outlet. PA1 introducing a first amount of oxygen-containing gas into the gaseous mixture; PA1 introducing the gaseous mixture into the reaction chamber; and PA1 introducing a further amount of oxygen or an oxygen-containing gas mixture into the reaction chamber through at least one secondary inlet located between the inlet and the outlet. PA1 (a) a gas distribution plate; PA1 (b) a fin block adjacent the gas distribution plate, the fin block comprising a base and a plurality of heat transfer surfaces extending from the base toward the gas distribution plate, the heat transfer surfaces joining with the gas distribution plate to form a labyrinthine channel therebetween, the labyrinthine channel having an inlet and an outlet, the base including in its interior portion at least one channel for circulating a thermal fluid therethrough for supplying heat to or removing heat from the reactor, thereby maintaining the reactor substantially isothermal, the labyrinthine channel having catalyst pellets disposed therein for promoting oxidation. PA1 (a) a gas distribution plate; PA1 (b) a fin block adjacent the gas distribution plate, the fin block comprising a base and a plurality of heat transfer surfaces extending from the base toward the gas distribution plate, the heat transfer surfaces joining with the gas distribution plate to form a labyrinthine channel therebetween, the labyrinthine channel having an inlet and an outlet, the base including in its interior portion at least one channel for circulating a thermal fluid therethrough for maintaining the reactor substantially isothermal, the labyrinthine channels having catalyst pellets disposed therein for promoting oxidation. PA1 (c) a first end plate adjacent the fin block on the side opposite the gas distribution plate; PA1 (d) a second end plate adjacent the gas distribution plate on the side opposite the fin block, the second end plate comprising a plurality of ports in fluid communication with the plurality of ports formed in the gas distribution plate; PA1 (e) a containment shell interposed between the first end plate and the second end plate, the containment shell circumscribing the fin block and the gas distribution plate; and PA1 (f) means for consolidating the containment shell, the first end plate and the second end plate into a pressure-tight assembly for containing the fin block and the gas distribution plate.
In the presence of carbon monoxide (CO), the oxidation of carbon monoxide is the prevalent reaction. This is believed to be so because the carbon monoxide is preferentially absorbed by the oxidizing catalyst, thereby effectively preventing the hydrogen (H.sub.2) from interacting with the catalyst to effect the other two undesired reactions (4) and (5) above. However, as the carbon monoxide (CO) becomes depleted along the length of the reaction chamber or catalyst bed, the hydrogen (H.sub.2) and oxygen (O.sub.2) react with the catalyst at a high rate (see reaction (4) above), thereby consuming the majority of the remaining oxygen. Then, in the oxygen-poor environment, the reverse water-shift reaction becomes prevalent, thereby increasing the concentration of carbon monoxide.
The oxidation of carbon monoxide (CO) is promoted by maintaining the temperature of the reactor within a desired range. Higher temperatures result in faster reaction rates, permitting the use of a smaller volume reactor but also promoting the undesired side reactions (4) and (5) above. Thus, an important factor in reactor design is to optimize the temperature of operation to provide a balance of activity (which affects reactor size) and selectivity (which affects the efficiency of eliminating carbon monoxide). At elevated reaction rates, effective heat exchange must also be provided so that the heat of reaction does not cause the reactor to overheat and adversely affect the balance of reactivity and selectivity (efficiency).
In the present selective oxidizing isothermal reactor, a fundamental component is the fin block, which is preferably formed from an aluminum casting containing a stainless steel tube coiled therein. The aluminum casting is machined in the general shape of a bowl, with the coil located at the base or bottom of the bowl.
The tubing coil, through which thermal fluid is directed, is preferably formed in the shape of two intermeshed spirals joined at their innermost ends, thereby forming a continuous path from the outside of one spiral, around the various coils of the first spiral, through a 180 degree bend into the innermost coil of the second spiral, around the various coils of the second spiral and finally exiting at the outside of the second spiral.
The concept of casting the coil into the aluminum block offers a simple, temperature-rated pressure boundary for thermal fluids. Other advantages include the good thermal conductivity and low density of aluminum, the low thermal resistance at the interface between the aluminum block and the stainless steel coil, and the accommodation of the differential thermal expansion coefficients between the two dissimilar metals.
As described in more detail below, the fin block is preferably bowl-shaped and contains a number of concentric circular grooves milled into the base of the bowl. Cylindrical aluminum fins are pressed into the grooves to create good thermal contact between the fin block and the fins. Each cylinder has a longitudinal gap, and is arranged such that the gap on each fin is 180 degrees from the gap on the adjacent fin(s). Thus, two continuous, symmetrical labyrinthine paths are formed from the smallest cylinder at the center of the bowl to the largest cylinder at the radially outermost portion of the bowl.
The volume between the fins forms the reaction chamber or catalyst bed, which is filled with pellets. In one embodiment, the active pellets comprise 0.3% platinum on alumina catalyst pellets interspersed with unplatinized, inert pellets in a ratio of about 1:2. The loading or concentration of catalyst on the active pellets can be varied along the length of the catalyst bed to achieve differing reaction rates. For example, the catalyst loading or concentration on active pellets can be gradually decreased to effect a lowering of the reaction rate in latter portions of the catalyst bed. In addition, the dilution of active pellets can be employed as a secondary mechanism to reduce catalyst concentration in order to control the reaction rate in any part of the catalyst bed, thereby insuring sufficient heat exchange capability to dissipate the heat of reaction.
When the fin block is loaded with catalyst, an aluminum air distribution plate is fastened over the top of the bowl, both to retain the catalyst pellets in the bed and to distribute the "secondary air". In a first embodiment, a number of secondary air injection ports are employed. The ports are located in orientations corresponding to selected gaps in the cylindrical fins. A closed-end porous stainless steel tube is preferably located inside the port to distribute oxygen-containing air along its length. Such a configuration provides a large surface area along which the air can enter the reactant stream, thereby promoting mixing and dilution prior to exposure to the catalyst.
In a second embodiment, oxygen-containing air is distributed at locations along the catalyst bed through a porous sheet formed of polytetrafluoroethylene (trade name "Teflon"). The air distribution plate on which the Teflon sheet is mounted has an annular recess on one face, undercut by a series of semicircular grooves. The grooves serve as air distribution channels which enable the air from the air inlet ports to be dispersed along the entire surface of the Teflon sheet (except for the area contacting semicircular ribs which support the Teflon sheet) installed within the annular recess. Upon assembly with the fin block, the Teflon sheet forms a semipermeable barrier between the air distribution channels and the catalyst bed, and induces a pressure differential across the Teflon sheet. The Teflon sheet also has the characteristic of enabling a predictable and repeatable gas flux in response to pressure differential. The Teflon sheet therefore evenly distributes air across the entire surface of the catalyst bed.
The aluminum fin block/air distribution plate assembly is encased in a pressure-rated housing. A stainless steel vessel secured with stainless steel flanges, tie rods and nuts is preferred. The aluminum reactor components are provided with clearance to expand both radially and axially within the stainless steel housing vessel in order to accommodate differences in the thermal expansion properties of the two metals. The tubing for the thermal fluid passes through the pressure boundary at some distance from the location at which it is embedded in the aluminum fin block in order to provide sufficient length of tubing to flex and absorb thermal deflections without imparting excess stress to the tubing.
It is envisioned that the selective oxidizing reactor will share a common, welded vessel with other reactors, such as the steam reformer and shift conversion reactor, in the overall fuel cell based power generation system.
The present isothermal reaction method and apparatus thus satisfy the following requirements:
The overall arrangement of the aluminum reactor inside a stainless steel shell provides the following advantages:
Accordingly, it is an object of the present invention to provide a method and apparatus for the selective oxidation of carbon monoxide to carbon dioxide in a hydrogen-containing gaseous mixture.
It is also an object of the invention to provide an apparatus and a method for the selective oxidation of carbon monoxide in a hydrogen-containing gas mixture by introducing oxygen or an oxygen-containing gas mixture at various locations along the reaction pathway to counteract the reverse water-shift reaction, which produces carbon monoxide in the absence of oxygen.
It is a further object of the invention to provide method for the selective oxidation of carbon monoxide over a wide range of reactant volume flow rates.
It is a still further object of the invention to provide a hydrogen-containing reformate gas mixture having carbon monoxide concentrations sufficiently low so as to be suitable for use in applications such as electrochemical fuel cells and other applications employing catalysts that would be adversely effected by higher carbon monoxide concentrations.